1. Field of the Invention
The present invention is directed generally to monitoring operating parameters in a steam turbine generator and, more specifically, to the monitoring of turbine blade vibration.
2. Description of the Prior Art
Turbine blades, because of their complex design, can suffer from vibration at frequencies which correspond to natural frequencies of the blades called modes. Each mode is associated with a different type of vibration such as along the rotational axis of the turbine, perpendicular to the rotational axis of the turbine, etc. To prevent excessive vibration of the blade about its normal position, normal design practice dictates that the blades be constructed such that those modes are located between harmonics of the operating frequency of the steam turbine. However, manufacturing tolerances, changes in blade attachment to the rotor, erosion, and changes in the operating frequency of the turbine, among other factors, cause mode frequencies to approach harmonics of the operating frequency. Additionally, damaging nonsynchronous vibration may also occur as a result of buffeting wherein a low steam flow and a high back pressure cause the random excitation of the turbine blades or as a result of turbine rotor torsional stresses.
The approach of the modes to the harmonics of the operating frequency may result in physical damage to the steam turbine. When the amplitude of the vibration exceeds a certain level, objectionable stresses are set up in the blade. If the condition is not detected and remedied, the blade may eventually fracture resulting in an extremely costly forced outage of the machinery.
One prior art technique for detecting blade vibration is to attach strain gages to the turbine blades. Because the strain gauges are attached directly to the blades, it does not matter if the blades are shrouded or unshrouded. Sensor information is communicated to analyzing equipment outside the machine by means of miniature transmitters affixed to the machine's rotating shaft at various locations. Although that technique is very accurate, it suffers from some significant drawbacks. First, the strain gauge has a short life due to erosion such that long term supervisory monitoring is not possible. Second, each blade requires a strain gauge if it is to be monitored. Because only a limited number of transmitters and, therefore, sensors can be accommodated inside the machine, only a few of the blades can be monitored at a time. Third, the complexity of continuously and reliably supplying power to the strain gage and transmitting the signal reliably from the rotating rotor disk to stationary electronics is very difficult.
To obviate those problems, apparatus exist for detecting turbine blade vibration which utilize permanently installed, non-contacting proximity sensors. One such apparatus is disclosed in U.S. Pat. No. 4,573,358 wherein a plurality of sensors spaced about the periphery of the blade row produce pulses whenever a blade passes a sensor. That pulse information is used to detect vibration of operator selected blades. However, the apparatus disclosed in that patent cannot be used on shrouded turbine blades because the shroud located at the periphery of the blades shields the blade tips from the sensors.
Another technique for monitoring blade vibration designed for monitoring the vibration of shrouded turbine blades is disclosed in U.S. Pat. No. 4,896,537. That patent discloses a shrouded turbine blade vibration monitor comprised of a plurality of sensors for sensing the profile of each of the turbine blade rows in a steam turbine by determining the presence of turbine blade shroud segments and turbine blade tenons. A processor is provided for storing an expected profile of the turbine blade row and then comparing the sensed profile to the expected profile to detect motion of the turbine blades due to vibration. Although that technique is effective, it is difficult to reliably sense the profile of the blade row under dynamic conditions. Thus, the need exists for easily and reliably generating, from a shrouded turbine blade row, simple blade pass sensor signals of the type used by unshrouded blade vibration monitors.